1. Field of the Invention
The present invention relates to a radar system in which a phased array antenna for transmission and a digital beamforming antenna for reception capable of forming multiple beams are utilized, and more particularly to a radar system with a transmission phased array antenna and a reception digital beam forming antenna adapted to observe targets present in a plurality of different directions in a transmission pulse repetition cycle.
2. Description of Prior Art
FIG. 1 is a block diagram illustrating such a prior radar system as disclosed in Japanese Patent Public Disclosure (Kokai) No. 63-167287 and Japanese Patent Public Disclosure (Kokai) No. 63-167288. In FIG. 1, numeral 101 designates a transmission pulse division and distribution circuit adapted to divide each of transmission pulses modulated as required and sent out from an oscillation circuit (not shown) into an optional number of sub-pulses, and to distribute and output such sub-pulses to respective phase shifters 2011 of first through n-th transmission and reception modules 201-20n. Division into sub-pulses as above mentioned is executed in accordance with information about a number of targets transmitted from a reception beam processing apparatus (not shown) of the radar system. Numeral 102 designates a transmission beam control circuit adapted to establish phase shifting amounts at respective phase shifters 2011 of the first through n-th transmission and reception modules 201-20n in respect of the divided respective sub-pulses. These phase shifting amounts are determined in accordance with information about direction and distance of targets transmitted from the reception beam processing apparatus (not shown).
The above-mentioned first through n-th transmission and reception modules 201 through 20n are arranged in relation to respective element antennas 301-30n of an array antenna 300, and control emission of transmission beams to the targets (not shown) through the respective corresponding element antennas and receive the waves reflected from the targets covered by the transmission beams respectively.
These modules will next be explained in detail by taking the first transmission module 201 as an example. The module 201 consists of the phase shifter 2011 adapted to execute a phase shifting process on the respective sub-pulses of sub-pulse train F.sub.1 supplied from the transmission pulse division and distribution circuit 101 in an amount corresponding to the instruction C.sub.1 provided from the transmission beam control circuit 102, a transmission amplifier 2012 adapted to amplify the sub-pulse which has been phase shifted, a circulator 2013 adapted to supply the amplified sub-pulse to the corresponding element antenna 301 so as to emit it to the target and take from the element antenna 301 a signal reflected from the target, a receiver 2014 adapted to phase-detect the reflected signal (high frequency signal) received through the element antenna 301 and circulator 2013 and separate the signal into the elements I (In phase) and Q (Quadrature) containing amplitude and phase information, and an A/D converter 2015 adapted to quantize the phase-detected signal in terms of the elements I and Q by converting them into digital signals. The digital signals (DI, DQ) which have been thus converted are sent to a distribution circuit 400 as reception data R.sub.1 from the modules 201.
Similarly, digital reception data R.sub.2 through R.sub.n from the second through n-th modules 202 through 20n are sent to the distribution circuit 400.
The distribution circuit 400 is adapted to combine the reception data R.sub.1 through R.sub.n from the respective modules 201 through 20n and distribute them as a set in an optional number "m" in this embodiment. The set of the reception data R.sub.1 through R.sub.n is transmitted to each of first through m-th beam forming circuits 501-50m.
The beam forming circuits 501-50m are adapted to control as desired the amplitude and phase of the reception data R.sub.1 -R.sub.n and thereby provide reception beams in the desired directions.
FIGS. 2(a) and (b) are block diagrams illustrating transmission and reception systems of a holographic radar as disclosed in Japanese Patent Public Disclosure (Kokai) No. 63-187180 as the other example of the radar system. Compared to the example shown in FIG. 1 wherein the array antenna is sphered for both transmission and reception, the present example includes two array antennas respectively for transmission and reception. In FIG. 2(a) illustrating the transmission system, numeral 4 designates a group of phase shifters, numeral 5 a transmitter, numeral 6 a beam steering computer, and numeral 7 a transmission array antenna. In contrast to FIG. 1, the group of phase shifters 4 correspond to the respective phase shifters 2011 of the first through n-th transmission and reception modules 201-20n. The transmitter 5 corresponds to the oscillation circuit and the like (not shown) in FIG. 1 and the transmission pulse division and distribution circuit 101. The beam steering computer 6 corresponds to the transmission beam control circuit 102 and the transmission array antenna 7 corresponds to the array antenna 300. In FIG. 2(b) illustrating the reception system, numeral 1 designates a reception array antenna, numeral 2 a local oscillation and distribution circuit, and numeral 3 a beam forming circuit. In contrast to FIG. 1, the reception array antenna 1 corresponds to the array antenna 300. The local oscillation and distribution circuit 2 corresponds to the respective receivers 2014 of the first through n-th transmission and reception modules 201-20n, the respective A/D converters 2015 and the distribution circuit 400. The beam forming circuit 3 corresponds to the first through m-th beam forming circuits 501-50m.
According to the radar system as shown in FIG. 1 or 2 wherein the phased array antenna for transmission and the digital beam forming antenna for reception capable of forming multiple beams are utilized, it is possible to observe the targets present in a plurality of different directions within a pulse repetition cycle T.sub.o by dividing a transmission pulse (shown in FIG. 3(a)) into the sub-pulses A, B, C, D and E as shown in FIG. 3(b) equivalent to the number of reception beams to be formed, continuously transmitting them in the respective target directions .theta..sub.A, .theta..sub.B, .theta..sub.C, .theta..sub.D and .theta..sub.E and receiving the reflected wave A', B'. . . from the targets in the non-transmitting period T.sub.F within the pulse repetition cycle T.sub.o as shown in FIG. 3(c).
According to this prior art, the number of the targets which can be treated or observed is limited to the number of the reception beams. Even if the number of reception beams may be increased, the pulse width of the transmission pulse, or a transmission period in the pulse repetition cycle T.sub.o has to be made wider due to an increase in the number of sub-pulses. In this case, since no pulses can be received during the transmission period due to the influence of transmission output energy, the pulse reflected from targets located near to the radar system cannot be received, and therefore, it is difficult to detect such targets.